You've got the tank, you've got the fish and the plants and the gravel, you've got the filters and the bacteria and the lights. What else did you need?

Well, have you looked at how well your water is moving? If you have a power filter, it might be helping, but the odds are it's not doing enough - you might need to look at adding a powerhead, a water director, or a wavemaker to add a little more action to your tank.

Here's a quick list of all the benefits of having a steady liquid "breeze" circulating through your whole tank:

A moving current will spread nutrients and oxygen to all of the fish, plants and bacteria in your tank. This will enhance your biological filtration, which reduces the amount of toxins floating in the water.

I always remember that moving water is really important in regulating temperature, keeping the water temperature stable by distributing heat evenly. I noticed that it can also prevent hot spots from developing near lights or motors, or cold spots in distant corners of the tank.

A current that moves through the whole tank will prevent the formation of "dead spots", where the water is stagnant and oxygen-poor, and where fish and food wastes collect and decay.
I hate these - spots like this increase the toxins in the water, foster algae growth, and make your water cloudy and disgusting. The moving water also stirs up debris and particles, so that they're suspended in the water where your mechanical filter can remove them, rather than settling in a scummy layer on your tank furniture and gravel.

Probably the most important benefit your aquarium gets from moving water is oxygenation. By disturbing the surface of the tank, it increases the amount of water exposed to the air, leading to a quicker exchange of gases. The water molecules release carbon dioxide wastes and absorb tons of oxygen. Richly oxygenated water is absolutely essential to keeping your fish, plants and bacteria healthy. Keeping the bacteria healthy, in turn, helps to maintain pH level, and keep ammonia and phosphate levels to a minimum.

And finally, maybe it's just my imagination, but I think my fish are actually healthier and happier when they get to test their muscles by swimming against a gentle current. To be honest, I just think it's more FUN for them to be in water that moves!